docs: def unresolve_filename(self, package_dir, filename):
          """Retrieves the probable source path from the output filename.  Pass
          in a .css path to get out a .scss path.
  
          :param package_dir: the path of the package directory
          :type package_dir: :class:`str`
          :param filename: the css filename
          :type filename: :class:`str`
          :returns: the scss filename
          :rtype: :class:`str`
          """
          filename, _ = os.path.splitext(filename)
          if self.strip_extension:
              for ext in ('.scss', '.sass'):
                  test_path = os.path.join(
                      package_dir, self.sass_path, filename + ext,
                  )
                  if os.path.exists(test_path):
                      return filename + ext
              else:  # file not found, let it error with `.scss` extension
                  return filename + '.scss'
          else:
              return filename